Forming an interference grating on a sample, and in particular on a photoconductive sample, makes it possible to activate the sample optically, and where appropriate to measure its properties.
Thus, U.S. Pat. No. 4,891,582 describes a technique for measuring the diffusion length LD of minority carriers in a photoconductive element.
That technique applies in particular to a thin layer photoconductive semiconductor that is for incorporating in photovoltaic modules.
Measuring this parameter makes it possible to determine the electronic quality of the thin film in order to verify whether it is suitable for being integrated in a photovoltaic module, i.e. whether it is suitable for obtaining good conversion efficiency.
Measuring the diffusion length LD of minority carriers in a thin film photoconductive semiconductor such as hydrogenated amorphous silicon is performed by laser interferometry.
The sample is constituted by a thin layer deposited on a substrate that is generally transparent and insulating and that has two parallel electrodes deposited thereon that are spaced apart by 1 millimeter (mm), for example. Electrical bias is applied between the two electrodes. A laser beam of vertically polarized light at a wavelength λ is split into two beams that are then deflected onto the sample so as to form a given angle.
An interference grating is developed between the electrodes and the pitch of the grating depends on the angle between the two beams. This illumination gives rise to a certain level of photocurrent Iw.
By using a λ/2 halfwave plate, the polarization of one of the two beams is changed over so as to replace the inference grating by illumination that is uniform. This illumination gives rise to a certain level of photocurrent Iw0.
The direction of the measured photocurrent is perpendicular to the fringes of the grating. Thereafter, variation in the ratio β=Iw/Iw0 as a function of the pitch of the grating is plotted, where the pitch is calculated in known manner from the angle between the two beams and a correspondence using a simple equation makes it possible to deduce from this curve the diffusion length of the minority carrier.
The link between β and LD is as follows. If the pitch of the grating is small, minority carrier diffusion eliminates the grating that has practically no influence on the measured current. The parameter β is then close to 1. If the pitch of the grating is large, minority carrier diffusion can no longer eliminate it. An array of space charges and of local fields is then put into place, thereby modifying the majority carrier current, and the parameter β may reach a value of −1.
This same space charge phenomenon lies behind the limit on current in a solar cell. In such a cell, if the minority carriers cannot diffuse, then the accumulation of minority carriers leads to a space charge being created that opposes the transport of majority carriers and that therefore gives rise to reduced efficiency. Thus, measuring LD on a thin film makes it easy to discover whether the film is suitable for integrating in a cell with significant chance of obtaining good efficiency prior to making the complete device, thus making such measurement most advantageous for all manufacturers of solar cells based on thin layers.
That technique is described in U.S. Pat. No. 4,891,582 and it applies to photoconductors and to semiconductors that are sensitive to light.
The implementation described in that document involves manual measurement, which is relatively difficult and lengthy to perform, since each step of the measurement, which corresponds to a different pitch of the interference grating, implies new adjustments.